In a column-sequential dot matrix printer, in which each character is reproduced by a series of sequentially printed columns of individual dots, with the print head advancing one step for each column, timing of the printer operations may be extremely critical. This is particularly true when the printer is operated at relatively high speeds, at which non-print functions of the mechanism, such as carriage return, line feed, tabulation, and other operations, require time intervals long enough to permit the accumulation of a number of new code words in the input storage for the printer. In one prior art print rate control system, which is particularly effective at moderate print rates, a two-speed arrangement is employed; a normal speed matched to the rate of incoming data is used when there is only one code word in the input register, and a slightly higher speed is employed when two or more code words are in storage. A system of this kind is disclosed in the aforementioned Fulton U.S. Pat. No. 3,938,641.
A more sophisticated and versatile print rate control system for a column-sequential dot matrix printer, which provides substantially improved operation at high speeds, is described in the above-mentioned Fulton application Ser. No. 608,586. In that system, the printing speed is controlled by at least three different print rate timing signals. One of these is a normal print rate signal of constant frequency that drives the print head at a speed essentially matched to the input rate of the data supplied to the printer. The second is a fast print rate timing signal of a substantially higher constant frequency. The third rate signal is of progressively increasing frequency, from an initial frequency substantially lower than the normal frequency to a final frequency at least as high as the normal frequency. The variable rate timing signal is used to accelerate the print head gradually to either the normal or the fast constant print rate, depending upon the number of code words in storage following each interval in which the print head has come to rest. When the printer is operating at the fast print rate and the number of code words in storage is reduced to a given level, the print head speed for succeeding cycles is dropped directly to the normal print rate.
Although the system of application Ser. No. 608,586 affords a substantial improvement in operation as compared to other prior print rate controls, including the system of U.S. Pat. No. 3,938,641, some difficulties and disadvantages are still experienced. Thus, it has been found that the system is not always sufficiently versatile in accelerating the print head at the beginning of a print line or in other circumstances in which the print head has been stopped. This difficulty is engendered by the fact that the maximum speed reached by the print head is the same for a wide variety of conditions ranging, for example, from two or three code words in storage to more than one hundred words in storage. If there are only a few words in storage, there is no necessity to accelerate the print head to an extremely high speed. On the other hand, if there is a really large number of code words in storage, the print head should be accelerated to a truly high speed, usually of the order of twice the normal print rate, in order to the printer to a reasonably current operating condition and avoid overrunning the capacity of the output storage register on a subsequent carriage return or like operation.
Known systems, if constructed to afford a fast operating speed that is substantially higher than the normal print rate, may permit unduly rapid deceleration of the print head when the number of code words in storage is reduced to the controlling level. This may result in a distortion of some of the printed characters, due to inertia of the printing mechanism. It may also produce undesirable wear characteristics, for the printing mechanism, over a long period of time, resulting from the abrupt changes that occur in the movement rate of the print head.